1. Field of the Invention
The present invention relates to a process for recovering actinides and/or lanthanides present in the trivalent state in an acid aqueous solution, particularly in a nitric solution.
2. District of the Prior Art
In installations for the reprocessing of irradiated nuclear fuels, it is standard practice to obtain in the first phase of the extraction of uranium and plutonium, aqueous solutions of fission products containing relatively large quantities of trivalent ions of lanthanides and actinides. The effluents from these installations also contain the same ions.
In view of the relatively long period of trivalent actinide elements, it is of considerable interest to separate these from nitric aqueous solutions, in order to obviate the handling of waste and effluents having a high .alpha. activity. Hitherto, this separation has been carried out by solvent extraction, using as extractants neutral or acid organophosphorus compounds. However, the use of such extractants has proved relatively unsatisfactory, because their extraction efficiency with respect to trivalent ions is low and their industrial use causes numerous problems, as a result of the need to work at a pH-value higher than 2 and the necessity of using large quantities of salifying agents, which increases the volume of the waste and the processing costs.
Consideration has also been given to the carrying out of said separation with other bidentate or neutral organophosphorus compounds, such as diphosphonates and carbamylphosphonates, together with amides, as described in J. Inorg. Nucl. Chem. 25, 1963, pp. 883-882, J. Inorg. Nucl. Chem. 26, 1964, pp. 1991-2003, and Separation Science and Technology, 15, (4), pp. 825-844, 1980.
However, the results obtained are not satisfactory, particularly with diamides such as tetrabutyl malonamide, because the extraction efficiency levels are low.
However, research has been carried out on other diamides, because there are certain advantages in using amides for the extraction. Thus, the processes used in the preparation of amides are generally simple and inexpensive, whilst their radiolytic and chemical stability is generally good. Moreover, amide degradation products (carboxylic acids and amines) do not disturb the extraction cycle and are easy to eliminate. Finally, the thermal decomposition of diamides causes no serious problems, unlike in the case of organophosphorus extractants.